Location based services for mobile stations are expected to play an important role in future applications of wireless systems. Some examples of such services are emergency services, road assistance, location based “yellow pages” and traffic information.
A wide variety of technologies for locating mobile stations have been developed. Many of these have been targeted towards the United States Federal Communication Commission (US FCC) requirement to determine the location of emergency 911 callers with high accuracy. These technologies may be classified into external methods or network based methods. An example of an external method is the Global Positioning System (GPS). The network based methods can be further categorized depending on whether it is the network or the mobile station that performs necessary signal measurements. The signal measurements themselves may involve the reception time of signals communicated between a base station BS and a mobile station MS, the angle of arriving signals or round trip delay measurements of signals communicated between a serving BS and an MS, or combinations thereof.
Most methods require specific hardware in the MS and/or in the network. Furthermore, Location Measurement Units (LMUs) are required for some methods to obtain knowledge about the relative time differences for sending signals to different mobile stations. This means that the operator is faced with an initial cost for investing in new equipment. This applies for both network and MS based methods.
For many location based services, it is expected that an accuracy of 500 m or even more is sufficient. For these types of services, investments in new expensive equipment is not easily justified. For some cases, a phased solution is the most attractive choice. The operator will then initially offer services based on low accuracy positioning methods and may later invest in new equipment as the revenues increase.
For these reasons, it is of interest to investigate what can be done with a minimum of network impact. The currently available network information with respect to MS location includes the identity of the serving cell, timing advance and measurement reports from the MS. The timing advance is an estimate of a signal propagation time and is used for calculating the distance between the serving BS and the MS. The MS measurement reports include measurements of the received signal strengths and identities of neighboring BS's as well as those of the serving BS.
Time of Arrival (TOA) measurements provide a propagation time of signals between an MS and a BS. Time Difference of Arrival (TDOA) measurements provide the difference of signal propagation time of signals between the MS and two different BS's. The measurements of the two BS's are then used for calculating the actual position of the MS. This procedure, using well-known geometric equations, is called triangulation.
In a TOA measuring procedure as illustrated in FIG. 1, a mobile station MS is capable of communication with at least three base stations BS1–BS3. In order to determine the position of the MS, the distance between the MS and each of the three BS's is measured using the TOA technique. The measured distance R1 of BS1 defines a circle C1 around BS1, and the MS is located somewhere on the circle C1. Likewise, distances R2 and R3 of BS2 and BS3 respectively, are measured for defining the corresponding circles C2 and C3. The intersection of the circles C1, C2 and C3 define the location of the MS. This technique is further described in the International Patent Application WO 99/21389, which is incorporated herein by reference.
In the Time Difference of Arrival (TDOA) measuring technique, the position determinations use TDOA calculations which are further based on Time of Arrival (TOA) measurements. In this method, the position of the mobile station is located at or near the point where a plurality of hyperbolic arcs cross over one another. Such a method is described in the International Patent Application WO 99/29130, which is incorporated herein by reference.
The two most common known positioning methods are the Down-link Observed Time Difference of Arrival method (DL-OTDOA) and the Up-link Time of Arrival method (UL-TOA). The DL-OTDOA method is based on measurements performed by the MS.
FIG. 2 illustrates how the DL-OTDOA method works. A mobile station is capable of communication with a serving base station SBS at a distance d0 and further with two neighboring base stations NBS1 and NBS2 at distances d1 and d2 respectively. OTDOAs of downlink signals received from two base stations define a hyperbola, which is illustrated with dotted lines in FIG. 2. The areas indicated outside the dotted lines represent measurement error margins. When three or more BSs are available, a plurality of hyperbolas can be defined and the MS will be located in the intersection of these hyperbolas, which is indicated as a black area including the respective measurement error margins. In order to compensate for any non-time aligned transmissions from the different BSs, the Real Time Differences (RTDS) must be known if the BSs are not time synchronized, e.g., to a global time reference. The RTDs can be obtained e.g. by having LMUs in at least some of the BSs.
UL-TOA works in a similar manner, although in this case, the BSs make measurements on uplink signals transmitted by the MS.
The accuracy of the method depends on, e.g., the accuracy of the measurements but also on the relative positions of the MS and the BSs involved. The accuracy can be very poor for some configurations, which is sometimes referred to as Geometrical Dilution of Precision (GDOP). Mathematically, GDOP is defined as the accuracy of the position fix divided by the accuracy of the measurements.
The object of the invention is to provide a simple method of locating mobile stations in connection with unsynchronized base stations without requiring Location Measurement Units (LMUs).